Bartonella (formerly Rochalimaea) henselae is a bacterium carried by domestic cats that casues a variety of disease syndromes in humans, depending on their immune status. In immunocompetent persons, bartonellosis usually manifests as a granulomatous lymphadenitis, commonly known as cat scratch disease. Cat scratch disease occurs in an estimated 22,000 people per year, with over 2,000 requiring hospitalizaiton. Signs and symptoms range from mild to severe. In immunocompromised individuals, bartonellosis can be life threatening, causing bacillary angiomatosis, bacillary peliosis, recurrent bacteremia, endocarditis, and other serious conditions. Cats are the most popular companion animal in the United States; 57 million pet cats reside in one third of the households in this country and numbers are increasing annually. The prevalence of B. henselae bacteremia in cats (and hence the number presumably capable of transmitting infection) is unknown, but surveys of pet, pound, and stray cats throughout the United States indicate that, in most parts of the country, 30 percent 60 percent of the cat population has antibodies against the organism. As cats are the only known reservoir for B. henselae, it is important to understand the biology of the organism in the cat. It is proposed to examine the pathogenesis of B. henselae within the cat, the humoral and cellular immune response of cats to infection with B. henselae and investigate transmission among the feline population. To address these questions, the following specific aims are proposed: (1) To develop improved diagnostic assays for B. henselae infection in cats. (2) To determine the kinetics of B. henselae infection and examine the humoral and cellular immune response to the agent in specific pathogen free (SPF) cats. SPF cats will be infected with a pure culture of B. henselae and clinical, immunologic, and bacteriologic parameters measured. (3) To determine the role of the cat flea (Ctenocephalides felis) in the inter cat transmission of B. henselae. Fleas will be fed infected blood using artificial and natural means to evaluate whether fleas are potential vectors or an infectious source of the organism. (4) To examine the role of vertical transmission in the cat in the biology of B. henselae. Experimentally and naturally infected and non infected pregnant queens will be examined bacteriologically and immunologically through gestation and their litters similarly examined during the postpartum period.